The methods of hydroformylation of an olefin to prepare aldehydes or alcohols having one more carbon atom than the initial olefin consist in reacting this olefin with a synthesis gas in the presence of a complex catalyst containing a metal selected from the series of transition metals. Of the latter, the metals in group VIII of the Periodic Table, namely, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum are used, in particular.
These metals may be used in the form of carbonyl metals, but it is known that complex combinations which contain at least one biphyllic ligand such as a phosphine in addition to the metal and carbon monoxide yield more linear products and make it possible to work at lower pressures. Such combinations are described, for example, in French Pat. No. 1,300,404 and U.S. Pat. No. 3,239,566.
The process may be carried out by introducing, into a reaction zone, a supply stream made up of a mixture of hydrogen and carbon monoxide, the olefinic charge which is to be hydroformylated, and the recycled catalyst, dissolved in a solvent or in the heavy reaction products. The reaction product may be recovered from the reaction medium by distilling a liquid current continuously drawn off from the reactor, while the heavy products containing the catalyst are recycled as mentioned above.
The reaction products may also be recovered by extracting them directly from the reaction medium by entraining them in a current of gas leaving the reaction zone; this gas may, for example, be excess systhesis gas. This process has the advantage of leaving the catalytic system where it is, but it can only conveniently be used for the hydroformylation of light olefins such as propene, butenes or pentenes.
In the particular case of the hydroformylation of propene, any one of these methods may be chosen.
Of the catalytic systems mentioned hereinbefore, one of those which gives the best results and has therefore been widely developed on an industrial scale is made up of a catalyst containing rhodium in a complex combination with carbon monoxide and a triarylphosphine. For a system of this kind, it is advantageous, particularly for obtaining good selectivity in linear products, to use high molar ratios of phosphorus to rhodium, these ratios having a minimum value of 10.
Hydroformylation of propene is generally effected at a temperature between about 60.degree. C. and 150.degree. C., temperatures of between 80.degree. C. and 125.degree. C. being most frequently used. The total pressure of hydrogen and carbon monoxide is fairly low, between about 1 to 40 bars, and the molar ratio H.sub.2 /CO is between about 1/1 and 20/1.
Under these conditions of hydroformylation, the activity of the catalyst is found to decrease in the course of time. This decrease may be fairly slight, less than a few percent of the initial activity, per day, but it increases with the temperature, and under certain conditions there may be reductions in the catalytic activity of several dozen percent after a few hours of reaction. This phenomenon is extremely troublesome, as in every case it means that, sooner or later, the catalytic charge has to be replaced or re-treated.
It is known that certain products adversely affect (are "poisons" to) hydroformylation catalysts. This is true of certain compounds containing sulphur such as COS, H.sub.2 S, thioethers, mercaptans, and the like, or halogens such as chlorine, for example. These compounds are cited, in particular, in Jurgen Falbe's book "Carbon Monoxide in Organic Synthesis", Springer-Verlag, New York, 1970, or in French Pat. No. 2,377,991. When carrying out a hydroformylation reaction, obviously attempts are made to eliminate these poisons from the catalyst. They may be introduced by the olefin which is to be hydroformylated or by the synthesising gas. However, it is found that, in spite of taking every precaution to avoid introducing any detectable trace of these poisons into the reaction medium, the activity of rhodium/triphenylphosphine catalytic systems as described hereinbefore decreases in the course of time.
This phenomenon is described in French Pat. No. 2,377,992, but its causes are still not fully understood. It might be supposed that undectectable traces of poisons, such as those mentioned hereinbefore, may reduce the activity of the catalyst, by an accumulative effect, particularly if the catalyst is kept in the reactor when the reaction products are drawn off in the gaseous phase. It might also be thought that this phenomenon is due to the slow, irreversible development of the catalytic complexes to inactive types, or to the formation of organic molecules capable of combining with the rhodium to form inactive complexes.
Although the cause or causes of this phenomenon are still not fully known, numerous solutions have been proposed to remedy it. In French Pat. No. 2,357,511 it is stated that the deactivated catalytic system returns to its initial activity when traces of oxygen are admitted into the reactor. According to French Pat. No. 2,377,991, the stability of the catalytic system is improved by adding an alkyl-diarylphosphine to the triphenylphosphine; or again, according to Belgian Pat. No. 854,403, the deactivation of the catalyst is attributed to the formation of ill-defined heavy products. These harmful compounds are eliminated by continuous intense stripping of the reaction medium.